The present invention generally relates to tracking servo circuits for rotary recording medium reproducing apparatuses, and more particularly to a tracking servo circuit for a rotary recording medium reproducing apparatus which obtains a reproduced information signal by reading and reproducing a recorded signal by a pickup reproducing element which scans over a recorded track on a rotary recording medium. The tracking servo circuit according to the present invention causes the pickup reproducing element to accurately follow and scan over an information signal recorded track so as to obtain a reproduced information signal of a satisfactory signal-to-noise (S/N) ratio, based on a relative level difference and a polarity of the relative level difference between first and second reference signals which are reproduced from both sides of the information signal recorded track.
Conventionally, a rotary recording medium (hereinafter simply referred to as a disc) is known on which an information signal is recorded on a spiral track or concentric tracks as variations in geometrical configuration. In such a disc, reference signals for tracking control are recorded on both sides of the information signal recorded track, with a predetermined period. The recorded surface of the disc is flat, and the disc has an electrode function. A tracking servo circuit is provided in a reproducing apparatus which reproduces the recorded information signal from the disc by use of a pickup reproducing element. The tracking servo circuit forms a tracking error signal by differentially amplifying detected levels of envelopes of the first and second reference signals which are discriminated and separated from the reproduced signal. The pickup reproducing element is displaced by a minute distance so as to minimize the tracking error with respect to the information signal recorded track, responsive to the tracking error signal. In other words, by noting that a relative level difference between detected levels of envelopes of the first and second reference signals represents the tracking error quantity and that a polarity of the relative level difference represents the direction of the tracking error, the tracking servo circuit is designed as a closed loop which controls the displacement of the pickup reproducing element so as to make the relative level difference become equal to zero.
The level of a reproduced RF signal which is obtained from the pickup reproducing element and is applied to the tracking servo circuit, varies due to the recorded wavelengths of the first and second reference signals, differing according to the radial position on the disc, the width, depth, and shape of the recorded pits on the disc which vary, and the like. Further, in a case where the pickup reproducing element is a reproducing stylus of the type which detects the variations in the electrostatic capacitance and comprises an electrode, the level of the reproduced RF signal greatly varies (by .+-.5 dB, for example) due to a change in the state in which the reproducing stylus makes sliding contact with the disc, the wear of the disc, the width of the electrode of the stylus, and the like. Accordingly, the levels of the first and second reference signals which are discriminated and separated from the reproduced RF signal, constantly fluctuate due to the level variations in the reproduced RF signal. For this reason, the detected levels of the envelopes of the first and second reference signals do not accurately represent the tracking error quantity. Thus, when these detected levels of the envelopes of the first and second reference signals are used as they are to form the tracking error signal which is applied to a mechanism which displaces the pickup reproducing element, it is impossible to carry out a stable tracking control, and a mistracking inevitably occurs.
Accordingly, a tracking control apparatus was proposed in a U.S. Pat. No. Re 31160 in which the assignee is the same as the assignee of the present application. According to this proposed tracking control apparatus, an automatic gain control (AGC) circuit is provided in a signal transmission path before a stage where the reproduced RF signal is applied to a differential amplifier within the tracking servo circuit, so as to compensate for the level fluctuations in the two kinds of reference signals. The levels with which the two kinds of reference signals are reproduced fluctuate according to the tracking error quantity of the reproducing element with respect to the information signal recorded track, and hence, the tracking servo cannot be carried out in a normal manner when an automatic gain control is performed to independently and constantly control the levels of the two kinds of reproduced reference signals to a constant level. Therefore, the proposed tracking control apparatus is designed to control a sum of the levels of the reproduced first and second reference signals, to a predetermined constant level.
The loop gain of the tracking servo circuit is set to a high value near a limit of the system, because the response of the pickup reproducing element becomes slower as the loop gain becomes smaller. Hence, an inconsistency in the loop gain introduces serious problems. In a characteristic curve which takes the level variations of the first and second reference signals within the reproduced RF signal versus the tracking error of the pickup reproducing element, it is known that the loop gain described above can be represented by the slope of the characteristic curve where there is no tracking error. The levels of the reproduced reference signals greatly fluctuate according to the wear of the disc, the width of the electrode of the reproducing stylus, and the like, and varies non-linearly in response to the tracking error quantity.
However, the proposed tracking control apparatus described before simply performs an automatic gain control so that the sum of the levels of the reproduced first and second reference signals constantly assumes a constant level. Thus, the loop gain varied according to the wear of the disc, the width of the electrode of the reproducing stylus, and the like. Moreover, due to the variation of the loop gain, the operation to compensate for the tracking error greatly varied. As a result, the loop gain had to be set to a value which was considerably low compared to an allowable upper limit value, by taking into account the variation in the loop gain. In addition, when the loop gain was set to a value in the vicinity of the allowable upper limit value, the tracking servo operation became unstable due to the variation in the loop gain.